Work around RubyGems TTY detection

[pirj]

Bundler is installed along with latest versions of rubygems-update and binaries seem to clash.
Example failures:

Installing RubyGems 3.1.1
  Successfully built RubyGem
  Name: bundler
  Version: 2.1.0
  File: bundler-2.1.0.gem
bundler's executable "bundle" conflicts with /home/travis/.rvm/rubies/ruby-2.6.3/bin/bundle
Overwrite the executable? [yN]

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

Presumably, RubyGems incorrectly detect that it's a non-interactive shell and don't fall back to default false.

RubyGems issue.
Recommended workaround.

[pirj]

The build is red as expected, @benoittgt is fixing that part in #2231
At least we've passed through [yN].